[NIXL][OOT platform] support nixl_connector with oot platform and other nixl_backend (#25121)

Signed-off-by: Chendi Xue <Chendi.Xue@intel.com>

docs/features/disagg_prefill.md

@@ -31,6 +31,12 @@ Now supports 5 types of connectors:
   --kv-transfer-config '{"kv_connector":"MultiConnector","kv_role":"kv_both","kv_connector_extra_config":{"connectors":[{"kv_connector":"NixlConnector","kv_role":"kv_both"},{"kv_connector":"SharedStorageConnector","kv_role":"kv_both","kv_connector_extra_config":{"shared_storage_path":"local_storage"}}]}}'
   ```
 
+For NixlConnector, you may also specify one or multiple NIXL_Backend. Such as:
+
+  ```bash
+  --kv-transfer-config '{"kv_connector":"NixlConnector","kv_role":"kv_both", "kv_buffer_device":"cuda", "kv_connector_extra_config":{"backend":["UCX", "GDS"]}'
+  ```
+
 - **OffloadingConnector**: enable offloading of KV data to CPU memory, customizing the CPU block size (in tokens) and number of blocks to allocate (per worker):
 
   ```bash

docs/serving/expert_parallel_deployment.md

@@ -193,7 +193,7 @@ For production deployments requiring strict SLA guarantees for time-to-first-tok
 
 1. **Install gdrcopy/ucx/nixl**: For maximum performance, run the [install_gdrcopy.sh](gh-file:tools/install_gdrcopy.sh) script to install `gdrcopy` (e.g., `install_gdrcopy.sh "${GDRCOPY_OS_VERSION}" "12.8" "x64"`). You can find available OS versions [here](https://developer.download.nvidia.com/compute/redist/gdrcopy/CUDA%2012.8/). If `gdrcopy` is not installed, things will still work with a plain `pip install nixl`, just with lower performance. `nixl` and `ucx` are installed as dependencies via pip.
 
-2. **Configure Both Instances**: Add this flag to both prefill and decode instances `--kv-transfer-config '{"kv_connector":"NixlConnector","kv_role":"kv_both"}`
+2. **Configure Both Instances**: Add this flag to both prefill and decode instances `--kv-transfer-config '{"kv_connector":"NixlConnector","kv_role":"kv_both"}`. Noted, you may also specify one or multiple NIXL_Backend. Such as: `--kv-transfer-config '{"kv_connector":"NixlConnector","kv_role":"kv_both", "kv_connector_extra_config":{"backend":["UCX", "GDS"]}'`
 
 3. **Client Orchestration**: Use the client-side script below to coordinate prefill/decode operations. We are actively working on routing solutions.
 

tests/v1/kv_connector/unit/test_nixl_connector.py

@@ -27,6 +27,7 @@ from vllm.distributed.kv_transfer.kv_connector.v1.nixl_connector import (
     KVConnectorRole, NixlAgentMetadata, NixlConnector, NixlConnectorMetadata,
     NixlConnectorWorker, NixlKVConnectorStats)
 from vllm.forward_context import ForwardContext
+from vllm.platforms.interface import Platform
 from vllm.sampling_params import SamplingParams
 from vllm.v1.attention.backends.flash_attn import FlashAttentionBackend
 from vllm.v1.outputs import KVConnectorOutput, ModelRunnerOutput
@@ -56,7 +57,7 @@ class FakeNixlWrapper:
     def get_reg_descs(self, caches_data, memory_type: str) -> list:
         return [str(uuid.uuid4()) for _ in caches_data]
 
-    def register_memory(self, descs) -> None:
+    def register_memory(self, descs, backends) -> None:
         pass
 
     def get_xfer_descs(self, blocks_data, memory_type: str) -> list:
@@ -855,3 +856,52 @@ def test_register_kv_caches(dist_init):
             assert block_len == expected_block_len, \
                 f"Block entry {i}: Expected block len {expected_block_len}, " \
                 f"got {block_len}"
+
+
+class FakePlatform(Platform):
+    device_type: str = "oot"
+
+    @classmethod
+    def get_nixl_supported_devices(cls) -> dict[str, tuple[str, ...]]:
+        """
+        Returns a mapping from device_type to a tuple of supported 
+        kv_buffer_device for nixl.
+        """
+        return {'oot': ('oot', )}
+
+    @classmethod
+    def get_nixl_memory_type(cls) -> Optional[str]:
+        """
+        Returns the nixl memory type for the current platform.
+        """
+        return 'VRAM'
+
+
+@pytest.mark.parametrize("kv_buffer_device, nixl_memory_type", [
+    ("oot", "VRAM"),
+])
+def test_kv_buffer_to_nixl_memory_types(dist_init, kv_buffer_device,
+                                        nixl_memory_type):
+    """
+    Test that register_kv_caches() passes the correct memory types from the
+    config to the nixl_wrapper.
+    """
+    vllm_config = create_vllm_config()
+    # Override the default memory types in the config
+    vllm_config.kv_transfer_config.kv_buffer_device = kv_buffer_device
+    from vllm.distributed.kv_transfer.kv_connector.v1.nixl_connector import (
+        _NIXL_SUPPORTED_DEVICE)
+    _NIXL_SUPPORTED_DEVICE.update(FakePlatform.get_nixl_supported_devices())
+
+    with patch("vllm.distributed.kv_transfer.kv_connector.v1.nixl_connector.NixlWrapper"), \
+         patch("vllm.distributed.kv_transfer.kv_connector.v1.nixl_connector.threading.Event"), \
+         patch("vllm.distributed.kv_transfer.kv_connector.v1.nixl_connector.threading.Thread"), \
+         patch("vllm.distributed.kv_transfer.kv_connector.v1.nixl_connector.current_platform", FakePlatform), \
+         patch("vllm.distributed.kv_transfer.kv_connector.v1.nixl_connector._NIXL_SUPPORTED_DEVICE", _NIXL_SUPPORTED_DEVICE):  # noqa: E501
+
+        # Create connector and replace its worker with a fake one for isolation
+        connector = NixlConnector(vllm_config, KVConnectorRole.WORKER)
+
+        # Verify get_reg_descs was called with the correct memory_type
+        assert connector.connector_worker.kv_buffer_device == kv_buffer_device
+        assert connector.connector_worker.nixl_memory_type == nixl_memory_type

vllm/distributed/kv_transfer/kv_connector/v1/nixl_connector.py

@@ -58,6 +58,12 @@ except ImportError:
     logger.warning("NIXL is not available")
     NixlWrapper = None
 
+try:
+    from nixl._api import nixl_agent_config
+except ImportError:
+    nixl_agent_config = None
+    logger.warning("NIXL agent config is not available")
+
 # Supported platforms and types of kv transfer buffer.
 # {device: tuple of supported kv buffer types}
 _NIXL_SUPPORTED_DEVICE = {
@@ -65,6 +71,8 @@ _NIXL_SUPPORTED_DEVICE = {
     "tpu": ("cpu", ),
     "xpu": ("cpu", ),
 }
+# support for oot platform by providing mapping in current_platform
+_NIXL_SUPPORTED_DEVICE.update(current_platform.get_nixl_supported_devices())
 
 
 class NixlAgentMetadata(
@@ -448,8 +456,15 @@ class NixlConnectorWorker:
         self.vllm_config = vllm_config
         self.block_size = vllm_config.cache_config.block_size
 
+        self.nixl_backends = \
+            vllm_config.kv_transfer_config.get_from_extra_config(
+                "backends", ["UCX"])
         # Agent.
-        self.nixl_wrapper = NixlWrapper(str(uuid.uuid4()), None)
+        non_ucx_backends = [b for b in self.nixl_backends if b != "UCX"]
+        config = nixl_agent_config(backends=self.nixl_backends) if len(
+            non_ucx_backends) > 0 and nixl_agent_config is not None else None
+
+        self.nixl_wrapper = NixlWrapper(str(uuid.uuid4()), config)
         # Map of engine_id -> {rank0: agent_name0, rank1: agent_name1..}.
         self._remote_agents: dict[EngineId, dict[int, str]] = defaultdict(dict)
 
@@ -486,11 +501,15 @@ class NixlConnectorWorker:
         # used when device memory can not be registered under nixl
         self.host_xfer_buffers: dict[str, torch.Tensor] = {}
         self.use_host_buffer = self.kv_buffer_device == "cpu"
+        # support for oot platform which can't register nixl memory
+        # type based on kv_buffer_device
+        self.nixl_memory_type = current_platform.get_nixl_memory_type()
+        if self.nixl_memory_type is None:
             if self.kv_buffer_device == "cuda":
                 self.nixl_memory_type = "VRAM"
             elif self.kv_buffer_device == "cpu":
                 self.nixl_memory_type = "DRAM"
-        else:
+        if self.nixl_memory_type is None:
             raise RuntimeError(
                 f"{self.device_type} with {self.kv_buffer_device} kv_buffer "
                 "is not supported.")
@@ -766,7 +785,7 @@ class NixlConnectorWorker:
         descs = self.nixl_wrapper.get_reg_descs(caches_data,
                                                 self.nixl_memory_type)
         logger.debug("Registering descs: %s", caches_data)
-        self.nixl_wrapper.register_memory(descs)
+        self.nixl_wrapper.register_memory(descs, backends=self.nixl_backends)
         logger.debug("Done registering descs")
         self._registered_descs.append(descs)
 

vllm/platforms/interface.py

@@ -604,6 +604,21 @@ class Platform:
 
         return _synced_weight_loader
 
+    @classmethod
+    def get_nixl_supported_devices(cls) -> dict[str, tuple[str, ...]]:
+        """
+        Returns a mapping from device_type to a tuple of supported 
+        kv_buffer_device for nixl.
+        """
+        return {}
+
+    @classmethod
+    def get_nixl_memory_type(cls) -> Optional[str]:
+        """
+        Returns the nixl memory type for the current platform.
+        """
+        return None
+
 
 class UnspecifiedPlatform(Platform):
     _enum = PlatformEnum.UNSPECIFIED